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\documentclass{chowto}

\title{A Brief Description of the Hall C Beamline}
\howtotype{reference}
\author{Chen Yan}
\category{beamline}
%\maintainer{Chen Yan}
\date{April 15, 2003}

\begin{document}

\begin{abstract}

Fundamental Hall C beamline optics, configuration and the major functions
is described.

\end{abstract}

\section{Conceptual Beamline Optics}

\subsection{Separation Section}

Hall C beamline starts from the Lambertson magnet (match point) after
the recombiner section. Usually, one places the original source there.
The Lambertson dipole and the following 1 meter BN dipole plus three
quads (3C01,02, and 03) between them form the first 3.2 degree separation 
from the straight orbit. An achromatic imaging (R16 = R26 = 0) is obtained
at the end of this section.

\subsection{The First Match SectionSecond Section}

The next four quads (3C04, 05, 06, and 07) transport the beam achromatically
and give a double waist in front of Hall C 34.3 degree arc.

\subsection{Arc Section}

The arc section consists of 8 3 meter dipole magnets and 8 qudas. It provides
34.3$^{\circ}$ bending angle and an achromatic imaging. Like the general achromat,
at the mid-point 3C12, beam has a double waist and large dispersion about
4 cm/${\%}$. At the end of the arc, the beam is recombined achromatically
at 3C17.

\subsection{The second match section}

After Hall C arc, the final three quads (3C18, 19, and 20A)provide adequate 
focusing property on either Hall C target or G$_{0}$ target. The beam size 
adjustment is done by varying them.

\subsection{Chicane}

Two Chicane dipole magnets (B$_{E}$ and B$_{Z}$) are located downstream
the second match section. The function of chicane is to generate additional
bending power in vertical plane to compensate the beam vertical offset caused
by 3 tesla Hermholtz coils suround polarized target.
\section{Major Function of Hall C Beamline}

\subsection{Optics Decoupling}

There are three double focusing locations along Hall C beamline. At each
double focusing point, the beam forgets its former behavior, therefore,
the beam optics tune at each section is decoubled with the previous.
Any local disturpancy cannot be transfered to the next section. This 
is the major requirement of optics tune in order to provide high
reliability and reproducibility.

\subsection{Achromat}

During experiments, users want not only to have a well-focused beam on the
target, but also to have the beam spot size and the incident angle are
independent to beam momentum change, is defined as a term "double 
achromatic focusing", i.e. R${16}$ = R${26}$ = 0. This function is executed
mainly by Hall C $34.3^{\circ}$ arc achromat.

\subsection{Compatible optics for Moeller and beam energy measurement}

Some users want to have continuous beam transportation when the Moeller
polarimeter is on. The 3 tesla Helmholtz coils give axial field only.
It doesn't effect the global beamline optics. The two Moeller quads
have certain few percent effect on the downstream optics. With fine
adjustment of 3C18, 19, and 20A, the effect can be reduced and a set of 
compromising tuning parameters can be found.\\

A new energy measurement optics will be verified in 2003 by CASA. The idea
is to use the first half of achromat (dispersive section) to determine the 
absolute beam energy and to use the second half achromat recombining beam
achromatically. If this is doable, the Hall C beamline transporation optics
will be greatly simplified by applying only optics model. The impact of
such modification is to make beam energy measurement on-line, no optics
alternation is necessary.    

\end{document}

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